Synchronous Generator Line Synchronization

Transcription

1 Synchronous Genertor Line Synchroniztion 1 Synchronous Genertor Line Synchroniztion Introduction One issue in power genertion is synchronous genertor strting. Typiclly, synchronous genertor is connected to the utility grid through the process of line-synchroniztion which mounts to generting voltges tht mtch the grid so tht the mchine cn be electriclly connected. Once the mchine is synchronized to the grid, the ctive power cn be controlled through the shft torque nd the rective power is controlled through the field current. In this experiment, the synchronous genertor is connected to fixed-voltge fixed-frequency source through line synchroniztion. The power genertion levels re set nd the rective power is controlled to produce the genertor "V-curves". Line Synchroniztion Figure 2 shows the set-up for line synchroniztion. The mchine is first driven to synchronous speed mechniclly using "prime mover" which could be n electric motor or turbine. A DC field current is estblished in the rotor so tht the mchine genertes n open-circuit voltge tht mtches the three-phse source. Before closing the line contctors, set of conditions must be met. Specificlly, the genertor nd line voltge must mtch in terms of 1. Frequency of the voltges 2. Mgnitude of the voltges 3. Phse of the voltge

2 Synchronous Genertor Line Synchroniztion 2 Note tht mtching the phse in three-phse system implies mtching the phse sequence. Under the conditions bove, the voltges cross the contctors will be zero. The frequency of the genertor is set by the prime mover mechnicl speed m. In prticulr, the mechnicl speed is djusted so to mtch the source frequency f e ccording to 2 m 2 fe (1) poles After mtching the frequency, the mgnitude of the open-circuit rmture voltges V, V b, nd V c re set to mtch the mgnitude of the line voltges V s, V bs, nd V cs. Since the open-circuit voltges re equl to the bck-emf, the mgnitude cn be set using the field current. Assuming the frequency is mtched ccording to (1) the -phse bck-emf is E f elf I f (2) 2 The finl step is mtching the phse of the generted voltge to the phse of the line voltges. The key to this is tht the frequency is not mtched exctly. Over long time period, the phse of the generted voltges will shift nd t some point be ligned with the line voltges. At this time, ll conditions re mtched nd the synchroniztion lmps will go drk. When ll three lmps re off, it is sfe to close the contctor nd connect the genertor to the line. The phse sequence must lso be mtched. If the genertor hs different phse sequence thn the line, the lmps will blink one t time. If this is the cse, two phses of the line voltge (on the left side of the contctor in Figure 1) cn be swpped. The per-phse synchronous genertor model is shown in Figure 2. Once the genertor is connected to the line, the synchronous speed will be fixed to the frequency by In RPM, the synchronous speed my be clculted s 2 s 2 fe (3) poles 120 f n e s (4) poles

3 Synchronous Genertor Line Synchroniztion 3 Genertor V-curves In stedy-stte opertion, the ctive power flow cn be controlled using the shft torque of the prime mover. The rective power cn be controlled by djusting the field current. From the mchine per-phse model of Figure 2, it cn be seen tht the output power is P V I cos( ) (5) 3 If the rmture resistnce is neglected, it cn be shown through power reltionships tht Therefore, for constnt power opertion X I cos( ) E sin( ) (6) s f I cos( ) C (7) 1 where C1 nd C2 re constnts. E f sin( ) C (8) 2 From the KVL eqution, the phse voltge is Eˆ f Vˆ j X Iˆ (9) s Using the informtion from (5-9), the circuit phsors cn be plotted for constnt power s the field current is vried. This is shown for three vlues of field current in Figure 3. Note tht the synchronous genertor cn operte with leding power fctor, unity power fctor, or lgging power fctor. Since the mgnitude of Ê f is proportionl to field current excittion, the leding power fctor condition is sometimes referred to s under-excited opertion nd lgging power fctor condition is referred to s over-excited opertion for the synchronous genertor. Note tht the current Î nd the voltge Ê f follow the lines of constnt power given by (7-8). If the rmture current mgnitude is plotted versus field current for severl power levels, the resultnt plots re the genertor V-curves shown in Figure 4. The points mrked, b, nd c on the upper curve correspond to the operting conditions in Figure 3. Note tht for P 0, the genertor is supplying or bsorbing only rective power. For the underexcited cse denoted by point d) in Figure 4, the generted rective power is negtive. This mens tht the mchine is bsorbing rective power nd is effectively operting s n inductor. For the over-excited cse denoted by point e), the generted rective power is positive mening tht the mchine is supplying rective power. For point e), the mchine ppers s cpcitor. This mode of opertion is sometimes used for rective power compenstion on power system. Opertion t point e) is referred to s synchronous condenser opertion; condenser being n old term for cpcitor.

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6 Synchronous Genertor Line Synchroniztion 6 Lbortory Softwre Figure 5 shows screen-shot of the softwre for this experiment, LbSynchronousMchineLineSynchroniztionTCP. The synchronous mchine rmture voltge, rmture current, ctive power, nd rective power re displyed s well s grphs of the rmture voltge nd rmture current. The field quntities re lso displyed nd control is built-in for djustment of the field current. After going through the line-synchroniztion procedure, the field current will be djusted nd the dt is sved t ech step for plotting the mchine V-curves. The stndrd "Add", "Cler", "Print", nd "Sve" buttons re included. Three V-curves will be obtined in this experiment nd cn be mrked by selecting power levels of "0W" (shown in Figure 5), "50W", or "100W". Figure 5. Screen shot of LbView softwre used for this experiment.

7 Synchronous Genertor Line Synchroniztion 7 Lbortory mchines Figure 6 shows the digrm of the motor test stnd used for this experiment. The synchronous mchine is ctully wound-rotor induction mchine. It will operte s synchronous mchine when DC current is supplied to the rotor. Besides the rotor windings ccessible from the connector box, the mchine lso hs short-circuited dmper windings on the rotor. Both sides of ech sttor winding (s, bs, cs, n, bn, nd cn terminls) re brought out on the connector box for connection in wye or delt. However, in this experiment, the mchine will be connected in wye. The rotor is wye-connected internlly, nd the three terminls re brought out (r, br, nd cr). Synchroniztion lmps re connected in-between the sttor windings nd the line connection (terminls, b, nd c). A three-phse switch bypsses these lmps when switched on. This setup cn be used for synchronizing the genertor to the line or for synchronous motor strting. The synchronous mchine is rted t 208 V (line-to-line rms), 60 Hz, 250 W. It is 4-pole mchine nd thus hs synchronous speed of 1800 RPM. The dc mchine rmture nd field terminls re vilble for connection (A1, A2, F1, nd F2). The mchine is designed to be shunt-connected (rmture nd field in prllel) nd is used to drive the synchronous mchine s genertor or bsorb mechnicl lod when the synchronous mchine is operting s motor.

8 Synchronous Genertor Line Synchroniztion 8 Lbortory Work Figure 7 shows the wiring digrm for this experiment. Strt by connecting the dc mchine s shunt-wound motor through meter chnnel. Note tht the rheostt is connected in series with the field. The rheostt should be turned ll the wy counter-clockwise s shown in Figure 7. Next, connect the Mgn-Power DC output to the synchronous mchine through meter chnnel s shown in Figure 7. Lstly, connect the synchronous mchine rmture (terminls, b, nd c) to the meter box s shown in Figure 7. Mke sure the switch on the connection box is in the OFF position. As lst step, connect the meter box to the 208 V line voltge. This is the terminls A, B, nd C directly below the Mgn-Power DC supply. Adpter plugs will be necessry to mke this connection. Keep in mind tht you re connecting into n energized source. For this reson, the switch on the synchronous mchine connection box must be OFF. After connecting into this source, the synchroniztion lmps on the connection box will light up. The next step is to estblish field current. Switch on the Mgn-Power supply nd strt the Line Synchroniztion (LbSynchronousMchineLineSynchroniztionTCP) experiment on the computer (in tht order). Increse the commnded field current to 2.5 A using the softwre interfce. Mke sure tht the field supply is working by noting the current on the computer screen. Leve the current setting t 2.5 A. Next, switch on the source pnel nd increse the vric to 15%. At this point, the DC motor should strt. If it does not, reduce the vric to zero, switch off the source pnel. Swp the field terminls F1 nd F2 nd try strting the motor gin. Once the motor is running, increse the voltge until the speed is very close to 1800 RPM (s mesured with the hnd-held tchometer). This will be bout 55% on the vric setting. At this point, ll three lmps should be blinking in synchronism t very low frequency. If the lmps re blinking one t time, then the phse sequence is incorrect. If this is the cse, reduce the motor speed to zero, switch off the source pnel, nd swp the "b" nd "c" terminls of the synchronous mchine. When you bring the motor up to 1800 RPM gin, ll lmps should blink t the sme time. With the lmps blinking slowly, throw the synchronous mchine switch t n instnt when ll of the lmps go off. This will bypss the lmps nd connect the synchronous mchine to the line. At this point, the synchronous genertor is running with no lod s evidenced by the power reding of nerly zero. Reduce the field current to 1.5 A. Set the Lod Size to 0W in the softwre nd obtin set of V-curves by incresing the field current from 1.5 A to 3.5 A nd logging the dt for ech point (by clicking Add). Reduce the field bck current to 1.5 A. The ctul power consumption will be nonzero nd will vry due to friction nd other losses. The next step is to generte some power nd send it into the line by chnging the DC motor torque. Increse the rheostt setting until the synchronous genertor power reds -50 W on the computer screen. Set the Lod Size to 50W in the softwre nd record nother set of V-curves. Be sure to djust the rheostt t ech point to keep the power constnt. Increse the rheostt gin so tht the genertor power reds -100 W. Set the Lod Size to 100W in the softwre nd record nother set of V-curves (gin djusting the rheostt to keep the power constnt).

9 Synchronous Genertor Line Synchroniztion 9 Switch off the source pnel circuit breker nd then reduce the source pnel vric to zero. Switch off the synchronous genertor nd the mchine will come to stop. Switch off the Mgn-Power power supply.

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11 Synchronous Genertor Line Synchroniztion 11 Clcultions nd Questions 1. Crete plot of ctive power nd rective power versus field current for ll three levels of loding. Mke seprte plot for ech lod level, but use the sme xes scles for ech plot. The ctive power remins constnt nd the rective power trnsitions from negtive to positive. Note tht the mesured ctive nd rective power re positive going into the mchine which is opposite of the wy it is defined in the model. For this reson, it is best to first multiply the mesured ctive nd rective powers by -1 before doing the clcultions. 2. Crete the V-curve plots for this genertor by plotting the rmture current mgnitude versus field current. This should be one plot with ll power levels. Also compute theoreticl V- curves using dt from the open-circuit nd short-circuit tests on the sme mchine. 3. Sketch the phsor digrms for three operting points logged in the 100 W test; one for leding power fctor, one for unity power fctor, nd one for lgging power fctor. For synchronous rectnce use the vlue obtined from open-circuit nd short-circuit tests. 4. For the 0 W V-curve dt, compute the effective vlues of inductnce nd cpcitnce for the lowest nd highest vlues of field current respectively.